Fix hwmon location for kernel >= 2.6.31 (closes sf: #2865628)

Fix hwmon location for kernel >= 2.6.31 (closes sf: #2865628)

In the linux kernel 2.6.31 and above, device data can either be in
/sys/class/hwmon/hwmonDEV or /sys/class/hwmon/hwmonDEV/device.
Just stat'ing for the latter doesn't work since it can exist but not contain
the required data (see https://bugs.launchpad.net/bugs/435571 for details).
The patch could be improved to keep in memory the right location of the data
on the user's system instead of trying each time, but, is it worth doing it?